Logging while drilling (LWD) techniques are well-known in the downhole drilling industry and are commonly used to measure various formation properties during drilling. Such LWD techniques include, for example, natural gamma ray, spectral density, neutron density, inductive and galvanic resistivity, acoustic velocity, and the like. Many such LWD techniques require that the standoff distance between the various logging sensors in the drill string and the borehole wall be known with a reasonable degree of accuracy. For example, LWD nuclear/neutron measurements utilize the standoff distance in the count rate weighting to correct formation density and porosity data. Moreover, the shape of the borehole (in addition to the standoff distances) is known to influence logging measurements.
Ultrasonic standoff measurements and/or ultrasonic caliper logging measurements are commonly utilized during drilling to determine standoff distance and therefore constitute an important downhole measurement. Ultrasonic caliper logging measurements are also commonly used to measure borehole size, shape, and the position of the drill string within the borehole. Conventionally, ultrasonic standoff and/or caliper measurements typically include transmitting an ultrasonic pulse into the drilling fluid and receiving the portion of the ultrasonic energy that is reflected back to the receiver from the drilling fluid borehole wall interface. The standoff distance is then typically determined from the ultrasonic velocity of the drilling fluid and the time delay between transmission and reception of the ultrasonic energy.
Caliper logging measurements are typically made with a plurality of ultrasonic sensors (typically two or three). Various sensor arrangements are known in the art. For example, caliper LWD tools employing three sensors spaced equi-angularly about a circumference of the drill collar are commonly utilized. Caliper LWD tools employing only two sensors are also known. For example, in one two-sensor caliper logging tool, the sensors are deployed on opposite sides of the drill collar (i.e., they are diametrically opposed). In another two-sensor caliper logging tool, the sensors are axially spaced, but deployed at the same tool face.
The above described prior art caliper LWD tools commonly employ either pulse echo ultrasonic sensors or pitch-catch ultrasonic sensors. A pulse echo ultrasonic sensor emits (transmits) ultrasonic waves and receives the reflected signal using the same transducer element. Pulse echo sensors are typically less complex and therefore less expensive to utilize. Pitch catch sensors typically include two transducer elements; the first of which is used as a transmitter (i.e., to transmit ultrasonic waves) and the other of which is utilized as a receiver (i.e., to receive the reflected ultrasonic signal). Pitch catch ultrasonic sensors are known to advantageously reduce, or even eliminate, transducer ringing effects, by substantially electromechanically isolating the transmitter and receiver transducer elements. They therefore tend to exhibit an improved signal to noise ratio (as compared to pulse echo sensors).
The above described caliper logging tools generally work well (providing both accurate and reliable standoff determination) when the drill string is centered (or nearly centered) in a circular borehole. In such instances the transmitted wave is essentially normal to the borehole wall, which tends to maximize the reflection efficiency at the receiver. In many drilling operations (e.g., in horizontal or highly inclined wells) the drill string can be eccentered in the borehole. Moreover, in certain formation types the borehole may have an irregular (e.g., elliptical or oval) shape. In these operations the transmitted ultrasonic waves are sometimes incident on the borehole wall at a non-normal (oblique) angle, which can result in reduced ultrasonic energy at the receiver. In some cases there may be blind spots at which the reflected waves are undetected by the sensor. In such cases, a portion of the borehole wall is invisible to the standoff sensor. Since standoff measurements are essential to interpreting certain other LWD data, these blind spots can have significant negative consequences (e.g., especially in pay zone steering operations).
Therefore, there exists a need for an improved caliper LWD tool and/or a caliper tool utilizing improved standoff sensors, particularly for use in deviated (e.g., horizontal) well bores in which the drill string is commonly eccentered (e.g., on bottom). Such a tool and/or sensors may advantageously improve the reliability of caliper LWD measurements.